1. Field of the Invention
This invention relates to conducted electromagnetic interference (EMI) testing and more particularly in a preferred embodiment to decoupling apparatus and methods used to determine EMI sources and receptors within electrical equipment.
2. Description of the Prior Art
Various techniques have been employed to detect, measure and then suppress EMI in sensitive electrical equipment or test items. Interference or susceptibility detection and measuring should be conducted with the test item operating as close to service conditions as possible. Also the test item normally is operated in its intended manner with anticipated inputs applied and its outputs normally loaded.
A problem exists with simulating service conditions and normal manner of operation. To bring a test item out of its normal operating environment and to place it on a bench for a test, an actual duplication of operation of the test item seldom occurs. For example, if in a vehicle equipped with an engine control system that includes a microcomputer as a controller, assume it is desired to use a particular portable mobile two-way radio. Discovering that the radio works when the engine is cutoff but doesn't work well while the engine is running, the engine control system is then removed from the vehicle and placed in a bench test environment for study. A simulator (support for the engine control system) is used to make the control system work as if it is in the vehicle. Also assume the system responds as if it is operating in the vehicle controlling what it is supposed to be controlling. Assume also, the engine control system generates the same interference that it was putting out before it was placed on the bench; but now the interference reacts with the simulator. By virtue of change in wiring, the coupling between harnesses, other components and the impedance and length of wires, the engine control system generally radiates and conducts a different amount of interference.
Efforts have been made to standardize bench test setups in order to gain data that approaches actual circumstances.
In prior bench tests under similar circumstances, line impedance stabilization networks (LISN's) have been recommended in a number of interference and susceptibility specifications, for insertion in power leads to offer something approaching a standard impedance to the radio frequency (RF) current from test items. The LISN's, as required by some military specifications introduce a standard 50 ohm power-source impedance for the test item so that conducted RF interference measurements can be compared to pass/fail limits without accounting for a source-impedance variable. However, in several LISN designs, a 5-microhenry coil is used, so the device is suitable for use from 150 KHz to 25 MHz. Over this range, the source-impedance varies from about 5 ohms at 150 KHz to 50 ohms at 25 MHz. It is not usable much above 25 MHz due to stray impedance. While it does furnish a standard impedance, it is not the impedance seen in the normal installation. It was never intended to be anything other than an A.C. power lines simulator.
Normally when trying to identify the potential of a device being an interference source, it is thought that this determination depends upon how you measured the interference emanating from the source. This implies that different test processes produced different results for the same interference source. Realizing the above conditions exist, efforts were made toward devising interference measuring technique that didn't depend upon how the interference was measured.
The present invention is an improved apparatus and method over the system as taught in U.S. Pat. No. 4,763,062 of the present assignee for analyzing EMI emissions and/or susceptibility of test items under bench test conditions. Currently, a series of resonators are configured and interconnected in a particular manner along with a shunt capacitance network to form a decoupling device. This decoupling device, usually disposed between the test item and its support system, provides a means for coupling dc and lower frequency signals needed for normal test item operations, but decouples essentially all other high frequency signals. Interfering signals from the test item confront essentially open circuit impedance with respect to the decoupling network input and the test item is forced to produce its highest noise voltage, the decoupling network acting as a worst case load with respect to the output of the test item.
Departing from the normal approach to noise studies of detecting, measuring and then suppressing EMI, using the decoupling network and testing for worst case interference parameters provide a repeatable scheme for determining worst case signals in any environment in which the test item may be employed. After establishing worst case interference parameters, modification techniques of test item circuit components and connectors may be employed to minimize the effect of generating EMI sources or to immunize susceptible receptors within the test item.
This improved apparatus contains a reduced number of resonators formed from a particular material and a single capacitor rather than a capacitance network. The apparatus of the present invention provides an extended frequency range at greater than 500 ohms impedance.